The present invention relates to a method for recording signals at high recording density and reproducing the recorded signals using laser beam, and an optical information recording medium used in the method.
Examples of a so-called read-only optical information recording medium for reproduction of signals using laser beams include optical disks referred to as a compact disk (CD), a laserdisk (LD) and a digital video disk (DVD).
Currently, DVDs can record signals at a higher density than any other read-only optical information recording media that are commercially available at present.
The read-only DVD is an optical disk having a diameter of 120 mm and a maximum user capacity of 4.7 GB per layer for recording. The substrate commonly is formed of a disk-shaped polycarbonate having a thickness of 0.6 mm and a diameter of 120 mm.
Information signals are reproduced by irradiation of laser beams with a wavelength of 650 nm or 635 nm (practically in a range of 630 nm to 670 nm because of errors). A tracking servo mechanism that maintains a reproduction laser beam at the center of a recorded single track utilizes phase difference tracking error signals for reproduction of a DVD (e.g., National Technical Report Vol. 32 No. 4, August, 1986, pp.72-80) (Version 1 of DVD-ROM specification).
Examples of an optical information recording medium capable of recording and reproducing signals using laser beam include a phase change type optical disk, a magneto-optical disk, an optical disk using a dye material or the like. Among them, in a recordable phase change type optical disk, a chalcogenide typically is used as a recording thin film material. Generally, the crystalline state of the recording thin film material is used as a non-recorded state. Signals are recorded by irradiating the recording thin film material with laser beam so as to melt and quench the recording thin film material so that the material becomes amorphous. On the other hand, in order to erase signals, the recording thin film is irradiated with laser beams at a power lower than that for recording so that the recording thin film becomes crystalline.
Furthermore, for the purpose of improving the recording density of a phase change optical disk, it has been proposed to determine the disk structure so that a phase difference between light reflected from the non-recorded portion and light reflected from the recorded portion is caused with respect to a wavelength xcex of the reproduction laser beam (e.g., Japanese Patent Publication Nos. 2773945 and 2661293, and Japanese Laid-Open Patent Publication (Tokkai-Hei) No. 6-4900). Compared with a general reflectance-difference reproducing structure, the phase difference reproducing structure provides better quality signals for reproduction even if the signals are recorded at high density.
Typically, a substrate having spiral or concentric circular grooves referred to as guide grooves is used to obtain tracking error signals for recording and reproducing signals on a recordable optical disk. More specifically, the tracking error signals can be obtained by, for example, a push-pull method or a 3-beam method by irradiation of the laser beam for recording and reproduction. Alternatively, a substrate with a staggered arrangement of pits referred to as wobble pits is used for a tracking servo mechanism by a track wobbling method (e.g., xe2x80x9cOptical Disk Technologyxe2x80x9d edited by Morio Ogami, published by Radio Technology Corp, pp86-97).
As described above, read-only DVDs can store signals at a higher density than any other commercially available optical information recording media. However, users cannot record arbitrary information in the read-only DVDs.
An object of the present invention is to provide a recordable optical information recording medium that can be reproduced by a reproducing device for read-only DVDs and has stable focus servo characteristics.
The characteristics required for a recordable optical information recording medium that can be reproduced by a reproducing device for read-only DVDs are as follows:
1. To record signals at a physical recording density equal to that of the read-only DVD (bit length: 0.267 xcexcm/bit; track pitch: 0.74 xcexcm; and modulation system of signals: 8/16, RLL (2,10)).
2. To obtain phase difference tracking error signals from the optical information recording medium in which signals are recorded.
3. To have a reflectance equal to that of the read-only DVD.
However, regarding item 3 of the reflectance, a recording medium with a lower reflectance can be used by introducing minor changes such as raising the reproduction gain of the reproducing device for DVDs, lowering circuit noise, or raising the output of reproduction laser beam.
Another object of the present invention is to provide a method for recording signals at a physical density equal to that of the read-only DVD on the optical information recording medium having the above-described characteristics and reproducing the recorded signals.
The present invention has the following features to achieve the above-described objects.
A first feature of a method for recording and reproducing information on an optical information recording medium of the present invention includes the steps of irradiating an optical information recording medium with laser beam based on an information signal, the optical information recording medium including at least a recording thin film that effects a phase change between an amorphous state and a crystalline state by the irradiation of the laser beam, the recording thin film formed on a disk-shaped substrate including guide grooves; thereby recording a desired signal by forming a record mark on the recording thin film while applying a tracking servo mechanism with a tracking error signal obtained from the guide groove of the substrate; and irradiating the optical information recording medium where the record mark is formed on the recording thin film with laser beam; thereby reproducing a signal while applying a tracking servo mechanism based on a tracking error signal obtained from the record mark.
A second feature of a method for recording and reproducing information on an optical information recording medium of the present invention includes the steps of irradiating an optical information recording medium with laser beam based on an information signal while rotating a substrate and moving a laser beam irradiation portion so that an interval between recorded signals in a radial direction is constant, the optical information recording medium including at least a recording thin film that effects a phase change between an amorphous state and a crystalline state by the irradiation of the laser beam, the recording thin film formed on a disk-shaped substrate including a specular recording region. This effects the phase change to form a record mark on the recording thin film so that desired record signals are recorded at a constant interval in the radial direction; the optical information recording medium where the record mark is formed on the recording thin film is irradiated with laser beams; thereby reproducing a signal while applying a tracking servo mechanism based on a tracking error signal obtained from the record mark.
The method for recording and reproducing information on an optical information recording medium of the present invention with the above-described features allows recording and reproduction of signals at a physical density equal to that of a read-only DVD.
A first feature of an optical information recording medium of the present invention includes at least a recording thin film that effects a phase change between an amorphous state and a crystalline state by irradiation of laser beams. The recording thin film is formed on a disk-shaped substrate including guide grooves having a groove depth d (nm). The groove depth d, a wavelength xcex1 (nm) of a laser beam for forming a record mark on the recording thin film based on an information signal and a refractive index n1 of the substrate at the wavelength xcex1 satisfy the relationship:
0.05xc3x97xcex1/n1xe2x89xa6d; 
the groove depth d (nm), a wavelength xcex2 (nm) of a laser beam for reproducing the record mark formed on the recording thin film and a refractive index n2 of the substrate at the wavelength xcex2 satisfy the relationship:
dxe2x89xa60.09xc3x97xcex2/n2; 
a phase xcfx861 of light reflected from the record mark and a phase xcfx862 of light reflected from a non-record mark region with respect to a laser beam at the wavelength xcex2 satisfy the relationship:
(2n+0.7)xc3x97xcfx80 less than xcfx862xe2x88x92xcfx861 less than (2n+1.3)xc3x97xcfx80, 
where n is an integer; and
an amplitude intensity I1 of light reflected from the record mark in the optical information recording medium and an amplitude intensity I2 of light reflected from the non-record mark region with respect to an incident laser beam at the wavelength xcex2 (nm) satisfy the relationship:
I1 less than I2. 
A second feature of an optical information recording medium of the present invention includes at least a recording thin film that effects a phase change between an amorphous state and a crystalline state by the irradiation of the laser beam. The recording thin film is formed on a disk-shaped substrate. A phase xcfx861 of light reflected from a record mark in the optical information recording medium and a phase xcfx862 of light reflected from a non-record mark region with respect to a wavelength xcex2 of the laser beam for reproducing a signal recorded on the optical information recording medium satisfy the relationship:
(2n+0.7)xc3x97xcfx80 less than xcfx862xe2x88x92xcfx861 less than (2n+1.3)xc3x97xcfx80, 
where n is an integer.
An amplitude intensity I1 of light reflected from the record mark in the optical information recording medium and an amplitude intensity I2 of light reflected from the non-record mark region with respect to incident laser beam at the wavelength xcex2 (nm) satisfy the relationship:
I1 less than I2. 
The optical information recording medium of the present invention with the above-described features allows recording of signals at a density equal to that of a read-only DVD and reproduction with a reproducing device for a read-only DVD and has stable focus servo characteristics.
In the above embodiment, the main elements constituting the recording thin film preferably include Ge and Te, and a ratio of Ge to Te (Ge: Te) in atomic weight is in the range from 45:55 to 55:45.